In a small saucepan, heat butter until just melted. Remove from heat, and cool for 10 minutes.

Beat egg in a small container with water. Add egg mixture and sugar to saucepan with melted butter. Heat on low-medium until just boiling. Take off heat, and stir in whiskey/bourbon (will bubble vigorously). Keep stirring until combined; let cool before serving.

Note: The sauce can be made ahead of time and will thicken as it cools. Reheat in the microwave for 10 seconds, stirring until combined again.